1. Field
The present invention relates to an optimization method and a lithographic cell.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The manufacture of an IC chip involves the fabrication of many layers. In order to generate a more detailed pattern a plurality of lithography or etch processing steps may be used in the manufacture of each layer: this is known as double patterning. The first of these is known as lithography-etch-lithography-etch and in this a first pattern is exposed and etched. A second pattern, with features located in the spaces between the features of the first pattern, is then exposed and etched. Thus a pattern of greater frequency can be generated. Another similar double patterning technique is known as lithography-freeze-lithography-etch. A pattern is exposed in the resist, which is then frozen. A second pattern can then also be etched into the resist and both patterns are then etched into the substrate. Another double patterning method is known as the spacer method. In the spacer method resist is put down on the substrate then spacers placed on top of the substrate. The resulting pattern etched into the substrate.
Different variables, such as the exposure dose, affect the resulting feature in an exposure step. However, in a multi-step process such as double patterning the variables from each process step also affect the characteristics of features in subsequent steps. Thus the variables controlling each of the process steps affect each other and the characteristics of the final feature.